1. Field of the Invention
The present invention relates to semiconductor storage devices which are used as mask ROMs.
2. Prior Art
A variety of semiconductor storage devices have been developed and the scale of integration of storage devices has increased in response to improvement in fine processing technologies. Large-scale semiconductor storage devices have increased resistance of internal wires, so delay of internal signals becomes large. Thus, in large-scale semiconductor storage devices it is difficult to perform high-speed access. An access time of a semiconductor storage device is the time which is required until data output is determined in accordance with an inputted external address. Normally, a cycle for inputting the external address is restricted by the access time, so a cycle time is set by adding a certain margin of time to the access time. In order to shorten the access time, a variety of measures have been employed. For example, bit-line precharging has been employed.
However, due to the existence of delay elements, it is difficult to further shorten the access time. Among those delay elements, is included delay due to word lines, delay due to operations of sense amplifiers and delay due to output capacity. In the case of mask ROMs and EPROMs which use NAND-type memory cells, large-scale integration may be easy, but high-speed operations are difficult to realize.